1. Field of the Invention
The present invention relates to a signal transmitting and receiving circuit of a digital subscriber line, especially to structures of an echo cancelling circuit and a hybrid circuit of the signal transmitting and receiving circuit.
2. Description of Related Art
Please refer to FIG. 1, illustrating a system block diagram of a prior art digital subscriber line (DSL). The digital subscriber line system 10 comprises a client signal transmitting and receiving circuit 105, a telecommunication loop 150, and a central office (CO) 160. The signal transmitting and receiving circuit 105 transmits data signals to the central office 160 or receives data signals from the central office 160 via the telecommunication loop 150. The signal transmitting and receiving circuit 105 comprises a signal transmitting module 110, a signal receiving module 120, an echo cancelling circuit 130, and a hybrid circuit 140. The signal transmitting module 110 generates data signals, which are transmitted to the hybrid circuit 140 by differential signals. The hybrid circuit 140 comprises transformers, which are used to couple the data signals onto the telecommunication loop 150 by electromagnetic coupling to transmit the data signals to the central office 160. When the central office 160 transmits data signals to the signal transmitting and receiving circuit 105, the data signals on the telecommunication loop 150 are also coupled to the signal receiving module 120 for further signal processing by electromagnetic coupling of the hybrid circuit 140.
When the signal transmitting module 110 transmits data signals, a part of the data signals will be coupled to the receiving end of the signal transmitting and receiving circuit 105 because the transmission and reception of the data signals are both processed by the hybrid circuit 140. That is, the data signals received by the signal receiving module 120 comprise the data signals transmitted by the signal transmitting module 110. To prevent the data signals from the signal transmitting module 110 from interfering the signal receiving module 120, which may cause loss of signal to noise ratio (SNR), the echo cancelling circuit 130 is provided to couple between a differential signal transmitting path 115 and a differential signal receiving path 116. The echo cancelling circuit 130 simulates characteristic impedance of the hybrid circuit 140 and the telecommunication loop 150.
Please refer to FIG. 2, illustrating the transmission of data signals inside the signal transmitting and receiving circuit 105. The data signal TX transmitted by the signal transmitting module 110 not only enters the hybrid circuit 140 for coupling to the telecommunication loop 150 but also enters the echo cancelling circuit 130. The data signal TX that enters the hybrid circuit 140 will generate an echo signal TXecho inside the signal transmitting and receiving circuit 105 because of electromagnetic coupling. The data signal RX received by the signal transmitting and receiving circuit 105 via the telecommunication loop 150 is coupled to the signal receiving module 120 via the hybrid circuit 140. As a result, the signal receiving module 120 will receive a mixed data signal RX+TXecho instead of the pure data signal RX because of the interference of the echo signal TXecho.
The echo signal TXecho will be affected by not only the characteristic impedance of the hybrid circuit 140, but also the characteristic impedance of the telecommunication loop 150 because of the electromagnetic coupling of the hybrid circuit 140. To cancel the echo signal TXecho, the echo cancelling circuit 130 must simulate the characteristic impedance of both the hybrid circuit 140 and the telecommunication loop 150. An echo cancelling signal TX′ echo will be generated after the data signal TX passes through the echo cancelling circuit 130, and the echo cancelling signal TX′echo will be added to the mixed data signal RX+TXecho by the adder 170 to form a data signal RX′ that is equal to RX+TXecho-TX′echo and will be received by the signal receiving module 120. If the echo cancelling circuit 130 is able to perfectly simulate the characteristic impedance of both the hybrid circuit 140 and the telecommunication loop 150, the echo signal TXecho and the echo cancelling signal TX′echo will be ideally equivalent, and thus the signal receiving module 120 can receive only the data signal RX.
The prior art echo cancelling circuit 130 is made of active components, which have advantages of forming any transfer function to easily simulate the polynomials representing the characteristic impedance of the hybrid circuit 140 and the telecommunication loop 150. However, because the active components have bandwidth limitations and zeroes and poles do exist, impedance matching conditions may not be very good within certain frequency ranges or the effects might be very bad at zeroes and poles.
Moreover, the transformers used by the prior art hybrid circuit 140 also have drawbacks. Please refer to FIG. 3, illustrating a part of the signal transmitting and receiving circuit 105 of the digital subscriber line system 10. Since it is now focused on the coupling part of the signal transmitting module 110 and the transformer 310 on the signal transmitting path inside the signal transmitting and receiving circuit 105, the echo cancelling circuit 130 and the signal receiving path are omitted for brevity. The transformer 310 comprises windings 313 and 316, having a turns ratio of N:M. The winding 313 comprises coils 314 and 315, and the winding 316 comprises coils 317 and 318. Because the voltage induced by the coils has polarity, the two ends of each coil 314, 315, 317, and 318 inside the transformer 310 are considered to be a first polarity end and a second polarity end. In this disclosure, a hollow circle is used to represent the first polarity end and the second polarity end has no marks on it. As shown in FIG. 3, a positive end TXP of the differential signal pair of the signal transmitting module 110 is coupled to the first polarity end of the coil 314 through the impedance unit 301, the second polarity end of the coil 314 is coupled to the first polarity end of the coil 315, and the second polarity end of the coil 315 is coupled to the negative end TXN of the differential signal pair of the signal transmitting module 110 through the impedance unit 302. On the other side of the transformer 310, the second polarity end of the coil 317 is coupled to the first polarity end of the coil 318 through the capacitor 319, and the two ends of the telecommunication loop 150 are respectively coupled to the first polarity end of the coil 317 and the second polarity end of the coil 318. Because the signal transmitting module 110, the impedance units 301 and 302 and the winding 313 are connected in series, if the signal transmitting module 110 wants the transmitting signal to have an optimal upstream power to transmit, according to the impedance matching theory, the output voltage of the signal transmitting module 110 must be twice the voltage required by the winding 313. In this way, the two ends of the winding 313 can obtain required voltage, but unfortunately half the voltage swing is lost on the impedance units 301 and 302. In addition, a higher working voltage is a burden to components that are manufactured by advanced semiconductor processes and thus are less endurable to high voltages. However, raising the voltage endurance of the components may increase the circuit cost and making the components work under too much voltage may shorten the life of the components.